1. Field of the Invention
The present invention relates to a method and an apparatus for determination of the coagulant injection rate (ratio of the amount of coagulant to be injected to the amount of water to be treated) in coagulation and sedimentation for treating surface running water such as river water, lake and marsh water, or industrial water, sewage, sludge, factory wastes, etc.
2. Background Art
A water purification plant employing a rapid filtration system generally comprises a mixing basin for rapid mixing with coagulant injection thereinto, a flocculation basin for growing the flocs formed in the mixing basin, a sedimentation basin for sedimenting and removing the grown flocs, and a filtration basin for removing the particles and flocs that could not be sedimented in the previous step (see FIG. 7 in Patent Reference 2).
The important point in the rapid filtration system is that the coagulant injection rate therein is controlled at a proper level in accordance with the quality of the raw water to be treated (untreated water), thereby forming well-sedimentable flocs. Coagulation treatment, if effected at an unsuitable coagulant injection rate, gives problems of loss head increase in filtration basin, backwash frequency increase, and outflow of particles from filtration basin, owing to carryover of flocs from sedimentation basin or coagulation failure.
A suitable coagulant injection rate varies depending on not only the turbidity of raw water but also the alkali degree, the pH and the temperature thereof, and therefore differs for different raw water; and accordingly, it is impossible to indiscriminately determine the coagulant injection rate on the basis of the raw water turbidity. Heretofore, therefore, the following methods have been employed in water purification plants for monitoring the coagulation condition and determining and controlling the coagulant injection rate.
(1) Jar Test:
The process is as follows: A constant amount of raw water to be treated is sampled in some beakers, the coagulant injection rate is stepwise varied in every beaker, the water in each beaker is coagulated through rapid mixing and slow mixing, then this is statically kept for a predetermined period of time, and thereafter the turbidity of the supernatant and the flocs sedimentation condition in each beaker are checked, thereby determining the coagulant injection rate (see FIG. 8 in Patent Reference 2).
These steps in the process are generally carried out by manual analytic operation; however, as in Patent Reference 1, an auto-jar tester has been put into practical use, which is for full-automatically attaining all the steps of raw water sampling, coagulant injection, determination of mixer rotation number and rotation time, and supernatant turbidity measurement (for its details, see Patent Reference 1).
(2) Injection Rate Formula:
This is for feed-forward control based on the injection rate formula that indicates the relation to a suitable coagulant injection rate with parameters of the turbidity, the pH, the alkali degree and the temperature of raw water. The injection rate is found according to an experimental method based on a jar test and on the turbidity of the precipitated water in actual plants. As an advanced modification of this system, there are known a technique of feedback control combination based on the measured value of precipitated water turbidity, and a technique of utilizing fuzzy and neuro control for approximation to operators' jar test results and running results in actual plants (see paragraphs [0006] and [0007] in Patent Reference 2).
(3) Coagulation Sensor:
This is a method for controlling the coagulant injection rate, in which light beams are applied to the flow of the fluid to be analyzed, and the mean particle size and the particle concentration of the flocs are obtained from the mean value of the transmitted light amount and the standard deviation thereof, and the coagulant injection rate is thereby controlled so that the mean particle size of the flocs could be on a suitable level, like the method of the invention disclosed in Patent Reference 2 (for its details, see Patent Reference 2).
The following Patent References 3 to 6 that disclose the techniques relating to the present invention are described below for convenience of explanation thereof.    Patent Reference 1: JP-A 2-114178    Patent Reference 2: Japanese Patent No. 3205450    Patent Reference 3: Japanese Patent No. 3672158 (corresponding to U.S. Pat. No. 6,184,983)    Patent Reference 4: Japanese Patent No. 2824164    Patent Reference 5: JP-A 10-311784 (laid-open publication of Patent Reference 3, corresponding to U.S. Ser. No. 09/037,431)    Patent Reference 6: JP-A 2002-90284
However, the coagulation condition confirmation method and the coagulant injection rate determination method described in the above have the following problems.
The jar test method of (1) requires operators well skilled in the art, and has a problem in that it may give different data depending on different operators. In addition, it takes a long time of 30 minutes or so for confirming the coagulation condition and for determining a reasonable coagulant injection rate; and therefore, it is difficult to frequently carryout the jar test, and the method is problematic in that the reflection of its data on the coagulant injection rate in actual plants may be delayed.
Use of an auto-jar tester for automation of jar test operation may greatly reduce the load of operators' work; however, the method still takes 30 minutes or so for obtaining the test data, and therefore could not solve the problem of long time lag.
In the method based on the injection rate formula of (2), the injection rate formula differs for different raw water, and therefore, the method requires injection rate formula control in every water purification plant. In addition, in this, there is not the guarantee of permanent usability of the injection rate formula. Specifically, when a dam is constructed on the upstream side of a water intake port, or when river bank construction works are made, or owing to the influence of a heavy rain, the relation between the quality of water and the optimum coagulant injection rate may be broken, and therefore the method has a problem in that it lacks site and time universality.
The method with a coagulation sensor of (3) enables real-time automatic control of a coagulant injection rate to give flocs having a suitable floc particle size, and this solves the operator problem and the time-lag problem in (1) and solves the universality problem in (2). However, the suitable floc particle size differs depending on the quality of raw water, and therefore, for automatic control of coagulant injection, a database must be previously formed for the relation between the raw water turbidity and the optimum floc particle size. Specifically, data with a coagulation sensor must be obtained through the four seasons, and the method is problematic in that it takes a lot of time before its practical use.
In the above, there are mentioned various problems in water purification plants; and needless-to-say, coagulation and sedimentation in treating industrial water, sewage and factory wastes also has similar problems.